Despite decades of pharmacological and pesticide intervention, Plasmodium remains the most prominent pathogenic protozoan, responsible for widespread morbidity and mortality in tropical regions of the world. Progress in the "post-genomic" era, utilizing whole genome techniques such as micro-arrays, proteomics and genome sequence annotation, is rapidly providing research targets, but ultimate success will necessitate a gene-by-gene pursuit of biological questions concerning newly identified drug and vaccine candidates. Via whole genome sequence analysis of Plasmodium and Cryptosporidium we are determining the provenance of extracellular domains likely arising in the parasitic adaptation of the pathogenic Apicomplexa. A select set of genes identified in this analysis share the presence of an LCCL domain and have striking multi-domain architectures composed of animal- and bacterial-derived extracellular adhesive modules. These genes are singularly remarkable among apicomplexan multi-domain extracellular proteins in that orthologs are conserved across the apicomplexan clade and thus these proteins are likely to serve a conserved biological role widespread in the pathogenic Apicomplexa. Transcript and protein expression studies indicate that the Plasmodium orthologs, herein termed PCCp1, PCCp2 and PCCp3, are expressed in gametocyte stages and during gametogenesis, and thus they are promising candidates for study of biological function during mosquito transmission. Preliminary protein localization studies in mature gametocytes indicate that PCCp proteins are trafficked to the parasite surface or a sub-membranous compartment prior to secretion during gametogenesis. It is a goal of this proposal to refine cellular localization via immunoelectron microscopy. As a means to investigate biological function, PCCp1, PCCp2 and PCCp3 genes will be disrupted in P. falciparum cultured parasites and in the rodent malaria model, P. berghei. An assessment of phenotype will include intraerythrocytic stages as well as investigation of a role in transmission to anopheline mosquitoes. Preliminary results with an antiserum recognizing PfCCp1 LCCL domains indicate transmission-blocking activity in mosquito membrane feeds and thus a component of this proposal is to determine the candidacy of the PCCp proteins in a multi-subunit transmission blocking vaccine (TBV).